This invention relates to photography, in which a photocell controls exposure time or aperture, or both.
The prior art has included various devices based on the original concept of a photo-voltaic cell exposed to the illumination of a whole scene to be photographed and which actuates a meter movement. The latter either gives an indication to an operator, or actuates an iris diaphram in the optical path of the lens to adjust the effective exposure to the illumination reflected from the scene.
Subsequent devices have employed electronic circuits between the sensor of illumination and the controlled optical element. This has allowed particular laws of control to be impressed upon the control function, such as logarithmic; also, combination with other electrically represented parameters; also, variation of the laws of control by relatively automatic means.
The methods and means for accomplishing electronic control have been varied. Some have employed an oscillator, the frequency of which is varied by the control parameters. Electronic counters have also been used. Also, delay circuits with the delay dependent upon the control parameters. A bridge circuit has often been employed for the photo-responsive element. Analog type circuitry has been usual, but digital type circuitry has been proposed.
Certain still cameras employ one photo-sensor directly viewing the total field of view, with another one viewing the same through the typically stopped-down lens. The two responses are combined for exposure control purposes. Also, one photo-sensor may be restricted to view only the central portion of the field of view; i.e., the object of principal interest.